DISK SCHEDULING& VARIOUS SCHEDULING ALGORITHMS

Course Title: PRINCIPLES OF OPERATING SYSTEMS

Course No. CSE213

Submitted by: Submitted to:

Rakesh soni Mrs. Sameep kaur

N1802 B30

CSE/5th semester

Acknowledgement

I would like to thank many peoples, but can only name a few here. First of all I would like to thanks my Teacher Mrs. Sameep kaur for guiding me, without which I would not have come to know about such wonderful field.

Then I would like to thanks all the people in Technologies department, faculty members. And with them I would also like to thanks the Lovely Professional University for providing with this kind of unique training were the student are given to work on real time projects.

I would like to thanks my friends Mr. kamalpreet cheema, Mr.Arshpreet singh saini AND Mr.Gaurav Sharma who have been with me in my hardships. All this is only possible due to my parent. So I am grateful to them for providing me with whenever and whatever was needed and for their moral support.

INDEX

Sr.no Topic page.no

1. OPERATING SYSYTEM 1-2

2. NEED OF OPERATING SYSTEM 2

3. DISK STRUCTURE 3-5

4. DISK SCHEDULING 5-6

5. TYPES OF SCHEDULING 6-12

5.1) FIFO: fair, but near random scheduling 7-8

5.2) SSTF: possible starvation 8-9

5.3) SCAN: favor requests for tracks near the ends 9-10

5.4) C-SCAN 10-11

5.5) LOOK 11-12

6. SELECTION OF SCHEDULING ALGO 12-13

7. REAL TIME DISK SCHEDULING 13

8. FUTURE SCOPE 13-14

9. REFRENCES

1. Operating system

Operating system is a program that manages the computer hardware. It also provides a

basis for application program and acts as an intermediary between the computer user

and the computer hardware. One of the responsibilities of the operating system is to use

the hardware efficiency. For the disk drives, meeting this responsibility entails having

fast access time and large disk bandwidth. The access time has two major

components .The seek time is the time for the disk arm to move the heads to the

cylinder containing the desired sector. The rotational latency is additional time for the

disk to rotate the desired sector to the disk head. The disk bandwidth is the total number

of bytes transferred, divided by the total time between the first request for service and

completion of the last transfer. We can improve both the access time and the bandwidth

by scheduling the servicing of disk I/O requests in a good order.

At the simplest level, an operating system does two things:

1. It manages the hardware and software resources of the system. In a desktop

computer, these resources include such things as the processor, memory, disk space,

etc. (On a cell phone, they include the keypad, the screen, the address book, the phone

dialer, the battery and the network connection.)

2. It provides a stable, consistent way for applications to deal with the hardware

without having to know all the details of the hardware.

The first task, managing the hardware and software resources, is very important, as

various programs and input methods compete for the attention of the central processing

unit (CPU) and demand memory, storage and input/output (I/O) bandwidth for their own

purposes. In this capacity, the operating system plays the role of the good parent,

making sure that each application gets the necessary resources while playing nicely

with all the other applications, as well as husbanding the limited capacity of the system

to the greatest good of all the users and applications.

The second task, providing a consistent application interface, is especially important if

there is to be more than one of a particular type of computer using the operating

system, or if the hardware making up the computer is ever open to change. A

consistent application program interface (API) allows a software developer to write an

application on one computer and have a high level of confidence that it will run on

another computer of the same type, even if the amount of memory or the quantity of

storage is different on the two machines.

Even if a particular computer is unique, an operating system can ensure that

applications continue to run when hardware upgrades and updates occur. This is

because the operating system and not the application is charged with managing the

hardware and the distribution of its resources. One of the challenges facing developers

is keeping their operating systems flexible enough to run hardware from the thousands

of vendors manufacturing computer equipment. Today's systems can accommodate

thousands of different printers, disk drives and special peripherals in any possible

combination.

2. Need of operating system

In earlier day’s user had to design the application according to the internal structure of

the hardware. Operating System was needed to enable the user to design the

application without concerning the details of the computer’s internal structure. In general

the boundary between the hardware & software is transparent to the user. There are the

several uses of the operating system and some as follow :

Providing security to users program.

Easy interaction between the human & computer.

Starting computer operation automatically when power in turned on.

Loading & scheduling users program.

Controlling input & output.

Controlling program execution.

Managing use of main memory.

3. DISK STRUCTURE

The Hard Disk is a bigger type of a CD-Rom with the same format of file Storage,

Tracks, Cylinders and Sectors. The Hard Disk Drive is originally of RAW format when its

a fresh copy. The data is recorded on different tracks which are concentric circles which

are placed on the surface of the platter and they are like a chain of rings. These hard

disks have thousands of tracks which are on the platters and while the hard disk’s

moving head moves over the inner to the outer part of the disk the data is accessed.

Each track can hold thousands of bytes of data in sectors which are a part of tracks.

Hard Disk Drive Structure

A disk is accessed as an array: sector 0 is the first sector of the top track of

the outmost cylinder. The next sectors of the same track are then ordered.

Then, sectors from the next track are ordered. After all the sectors of all the

tracks are ordered, we move to the next cylinder. The innermost is the last

cylinder.

Magnetic disks provide bulk of secondary storage of modern computers.the following

terms are used to explained the working curiosity of disk.

– Transfer rate is rate at which data flow between drive and computer

– Positioning time (random-access time) is time to move disk arm to desired cylinder

(seek time) and time for desired sector to rotate under the disk head (rotational latency)

– Head crash results from disk head making contact with the disk surface

Magnetic disk strucure

Disk drives are addressed as large 1-dimensional arrays of logical

blocks, where the logical block is the smallest unit of transfer.

The 1-dimensional array of logical blocks is mapped into the sectors

of the disk sequentially.

1. Sector 0 is the first sector of the first track on the outermost

cylinder.

2. Mapping proceeds in order through that track, then the rest of

the tracks in that cylinder, and then through the rest of the cylinders from

outermost to innermost.

4. DISK SCHEDULING

When a process wants to do disk I/O, it makes a call to the operating system. Since the

operation may take some time, the process is put into a blocked state, and the I/O

request is sent to a part of the OS called a device driver. If the disk is idle, the operation

can be started right away, but if the disk is busy servicing another request, it must be

added to a queue of requests and wait its turn. Thus the total delay seen by the process

has several components:

The overhead of getting into and out of the OS, and the time the OS spends

fiddling with queues, etc.

The queuing time spent waiting for the disk to become available.

The latency spent waiting for the disk to get the right track and sector.

The transfer time spent actually reading or writing the data.

The operating system is responsible for using hardware efficiently for the disk drives,

this means having a fast access time and disk bandwidth

Access time has two major components

1. Seek time is the time for the disk are to move the heads to the

cylinder containing the desired sector.

2. Rotational latency is the additional time waiting for the disk to rotate

the desired sector to the disk head.

Minimize seek time

Seek time » seek distance

Disk bandwidth is the total number of bytes transferred, divided by the total time

between the first request for service and the completion of the last transfer.

5. Types of scheduling

The processes running on a machine may have multiple outstanding requests for data

from the disk. In what order should requests be served and by which algo, Although It is

mentioned that in a “queue” of requests, there is no reason why the requests have to be

satisfied first-come first-served. In fact, that is a very bad way to schedule disk requests.

Since requests from different processes may be scattered all over the disk, satisfying

them in the order they arrive would entail an awful lot of jumping around on the disk,

resulting in excessive rotational latency and seek time -- both for individual requests and

for the system as a whole. Fortunately, better algorithms are not hard to devise , types

of disk scheduling algos and their policies are as follow:

FIFO: fair, but near random scheduling

SSTF: possible starvation

SCAN: favor requests for tracks near the ends

C-SCAN

LOOK

Several algorithms exist to schedule the servicing of disk I/O requests and the following

example or query of illustrating the request according to the scheduling algos:

Query: Illustrate these with a request queue (0-199)

98, 183, 37, 122, 14, 124, 65, 67

5.1 First-Come-First-Served (FCFS)

FIFO is an acronym for First In, First Out, an abstraction in ways of organizing and

manipulation of data relative to time and prioritization. This expression describes the

principle of a queue processing technique or servicing conflicting demands by ordering

process by first-come, first-served (FCFS) behaviour: what comes in first is handled

first, what comes in next waits until the first is finished, etc. This is how nachos works

right now. As processes arrive, they queue up for the disk and get their requests served

in order. In current version of nachos, queueing happens at the mutex lock .

Drawback:- May have long swings from one part of disk to another. It makes sense to

service outstanding requests from adjacent parts of disk sequentially.

Answer of query in FCFS and example of FCFS

5.2 SSTF(Shortest Service Time First)

In this the Disk scheduler looks at all outstanding disk requests, and services the one

closest to where the disk head currently is. Sort of like Shortest-Job-First task

scheduling. It Selects the request with the minimum seek time from the current head

position. SSTF scheduling is a form of SJF scheduling; may cause starvation of some

requests. It select the disk I/O request that requires the least movement of the disk arm

from its current position. it always choose the minimum seek time and new requests

may be chosen before an existing request. The shortest seek first algorithm has the

direct benefit of simplicity and is clearly advantageous in comparison to the FIFO

method, in that overall arm movement is reduced, resulting in lower average response

time.However, since the buffer is always getting new requests, these can skew the

service time of requests that may be farthest away from the disk head's current location,

if the new requests are all close to the current location; in fact, starvation may result,

with the faraway requests never being able to make progress.

Drawback:- in this , Starvation. A request for a remote part of the disk may

never get serviced.

Example of SSTF with solution of query

5.3 SCAN

The elevator algorithm (also SCAN) is a disk scheduling algorithm to determine the

motion of the disk's arm and head in servicing read and write requests.

This algorithm is named after the behavior of a building elevator, where the elevator

continues to travel in its current direction (up or down) until empty, stopping only to let

individuals off or to pick up new individuals heading in the same direction.

From an implementation perspective, the drive maintains a buffer of pending read/write

requests, along with the associated cylinder number of the request. Lower cylinder

numbers indicate that the cylinder is closest to the spindle, and higher numbers indicate

the cylinder is further away. When a new request arrives while the drive is idle, the initial

arm/head movement will be in the direction of the cylinder where the data is stored,

either in or out. As additional requests arrive, requests are serviced only in the current

direction of arm movement until the arm reaches the edge of the disk. When this

happens, the direction of the arm reverses, and the requests that were remaining in the

opposite direction are serviced, and so on.It reduces the variant as compare to SSTF.

Example and answer of query scheduling in SCAN :

5.4 C-SCAN (CIRCULAR SCAN)

Both SCAN and C-SCAN behave in the same manner until they reach the last track

queued. For the sake of this example let us assume that the SCAN algorithm is

currently going from a lower track number to a higher track number (like the C-SCAN

is doing). For both methods, one takes the difference in magnitude (i.e. absolute

value) between the next track request and the current track. It Provides a more

uniform wait time than SCAN. The head moves from one end of the disk to the other.

It servicing requests as it goes. When it reaches the other end, however, it

immediately returns to the beginning of the disk, without servicing any requests on the

return trip. It Treats the cylinders as a circular list that wraps around from the last

cylinder to the first one.

Example with query solution sequence :

5.5 LOOK

LOOK is a disk scheduling algorithm used to determine the order in which new disk

read and write requests are processed. LOOK is similar to SCAN in that the heads

sweep across the disk surface in both directions performing reads and writes. However,

unlike SCAN, which visits the innermost and outermost cylinders each sweep, LOOK

will change directions when it has reached the last request in the current direction . One

variant of LOOK is C-LOOK, which fulfills requests in one direction only. That is, C-

LOOK starts at the innermost cylinder requested and moves outward fulfilling requests

until it reaches the last request. Then it moves back to the innermost request again.

LOOK has slightly better average seek times than SCAN. C-LOOK has a slight lower

variance in seek time than LOOK since the worst case seek time is nearly cut in half.

Examlpe and answer of query sequence

6. SELECTION OF SCHEDULING ALGO

Disk scheduling is the problem of deciding which particular request for data by

your computer from yourhard drive (or other storage medium) should be serviced first.

In other words, when several programs are trying to pull data from your hard drive at

once, which one gets the data first? It is a fundamental problem in operating

system scheduling in terms of minimizing the wait for the user when he or she simply

wants to do some things that require information from the hard drive.

SSTF is common and has a natural appeal

SCAN and C-SCAN perform better for systems that place a heavy load on the

disk.

Performance depends on the number and types of requests.

Requests for disk service can be influenced by the file-allocation method.

The disk-scheduling algorithm should be written as a separate module of the

operating system, allowing it to be replaced with a different algorithm if

necessary.

Either SSTF or LOOK is a reasonable choice for the default algorithm.

7. Real time disk scheduling

Previous real-time disk scheduling algorithms assume that each disk request incurs a

disk mechanical operation and only consider how to move the disk head under real-time

constraints. However, with the increased capacity of on-disk cache, modern disk drives

read-ahead data aggressively. Thus, the on-disk cache may service lots of requests

without incurring physical disk access. By exploring the design methodology of on-disk

cache, in this paper, we propose CARDS: a cache-aware real-time disk scheduling

algorithm that takes the on-disk cache into consideration during scheduling. Therefore,

the scheduling algorithm can help to minimize the cache miss ratio. Besides, the service

timing estimation is more accurate in schedulability analysis since the cache effect is

considered. A simulation-based evaluation shows CARDS to be highly successful as

compared to the classical real-time disk scheduling algorithms. For example, under

sequential workload with 10 sequential streams, the data throughput of CARDS is 1.1

times of DM-SCAN.

8. FUTURE SCOPE

Disk scheduling is an operating system process to service disk requests. It has an

important role in QOS guarantee of soft real-time environments such as video-on-

demand and multimedia servers. Since now, some disk scheduling algorithms have

been proposed to schedule disk requests in an optimized manner. Most of these

methods try to minimize makespan by decreasing the number of disk head seeks as

one of the slowest operations in modern computers and crucial for system performance

because it usually takes some milli-seconds. In this paper, we propose a new disk

scheduling method based on genetic algorithm that considers makespan and number of

missed tasks simultaneously. In the proposed method, a new coding scheme is

presented which employs simple GA procedures such as crossover and mutation and a

penalty function in fitness. To get the best performance of the proposed method, its

parameters such as number of chromosomes in initial population, mutation, and

crossover probabilities, etc have been adjusted by applying it on some sample

problems. The algorithm has been tested on several problems and its results were

compared with well-known related methods. Experimental results showed that the

proposed method worked very well and excelled most related works in terms of miss

ratio and average seeks

Proportional-share algorithms are designed to allocate an available resource,

such as network, processor, or disk, for a set of competing applications in proportion to

the resource weight allotted to each. While a myriad of proportional-share algorithms

were made for network and processor resources, little research work has been

conducted on disk resources, which exhibit non-linear performance characteristics

attributed to disk head movements. This paper proposes a new proportional-share disk-

scheduling algorithm, which accounts for overhead caused by disk head movements

and QoS guarantees in an integrated manner. Performance evaluations via simulations

reveal that the proposed algorithm improves I/O throughput by 11–19% with only 1–2%

QoS deterioration.

References

1. http://www.springerlink.com/content/tpw5bumjbjdkt6w7/

2. http://www.slideshare.net/J.T.A.JONES/disk-scheduling

3. http://www.ecs.umass.edu/ece/koren/architecture/Disk/help.htm

4. http://en.wikipedia.org/wiki/LOOK_algorithm

5. http://portal.acm.org/citation.cfm?id=98949.99156

6. http://everything2.com/title/disk+scheduling